1. Field of the Invention
The present invention relates to an optical device.
2. Description of the Prior Art
First of all, the configuration and operation of a conventional optical device are explained with reference to the FIGS. 13 to 16.
FIG. 13 is a drawing showing the optical system of a conventional optical device. As shown in FIG. 13, a diffraction grating 2 for producing three light beams, a holographic element 3 serving as a deflection member, a collimator lens 4 serving as a focussing member, and an objective lens 5 serving as a focussing member are arranged in that order on the optical axis of a light beam emitted from a semiconductor laser element 1 serving as a light-emitting element.
An information recording medium 6 is arranged in the focussing plane of the objective lens 5. Photodetector groups 7 with a plurality of photodetectors for receiving light that has been deflected by the holographic element 3 are arranged on both sides of the semiconductor laser element 1.
As shown in FIG. 14, a parallel grating with constant pitch extending in the X-axis is formed on the diffraction grating 2. Moreover, a diffraction grating (not shown in the drawing) functioning as a lens is formed on the holographic element 3.
The following is an explanation of the propagation of light emitted from the semiconductor laser element 1. As shown in FIG. 13, when passing the diffracting grating 2, the light emitted from the semiconductor laser element 1 is diffracted by the diffraction grating 2 in the direction of the Y-axis, and split into three light bundles, namely a +1-order diffraction beam, a xe2x88x921-order diffraction beam and a zero-order diffraction beam. It should be noted that, since the +1-order diffraction beam and the xe2x88x921-order diffraction beam are diffracted in the Y-axis direction, that is in directions perpendicular to the plane of FIG. 13, they cannot be portrayed in distinction the zero-order diffraction beam in this drawing. The zero-order diffraction beam is also referred to as the main beam, and is used to obtain the signal recorded on the information recording medium 6 and the focus error signal regarding the focus error between the optical device and the information recording medium 6. The xc2x11-order diffraction beams are referred to as sub-beams, and are used to obtain the tracking error signal. After passing the holographic element 3, these three light bundles are irradiated onto the collimator lens 4. The collimator lens 4 collimates the light beams into parallel light, and the objective lens 5 focuses the collimated light onto the information recording medium 6, where it is reflected back towards the objective lens 5.
Then, the light reflected by the information recording medium 6 propagates along the same path in the opposite direction, that is, through the objective lens 5, the collimator lens 4, and the holographic element 3, in that order. The light beams irradiated (again) onto the holographic element 3 are deflected in the X-axis direction, and enter the photodetector groups 7. The photodetector groups 7 receive the main beam and the sub-beams, and a calculating element (not shown in the drawings), which is connected to the photodetector group 7, calculates the signal recorded in the information recording medium 6, the focus error signal, and the tracking error signal.
The zero-order diffraction light in the region 11 of FIG. 14 enters the collimator lens 4, the +1-order diffraction light in the region 12 enters the collimator lens 4, and the xe2x88x921-order diffraction light in the region 13 enters the collimator lens 4.
In this conventional optical device, the diffraction efficiency can be set to an appropriate value by adjusting the diffraction grating depth of the diffraction grating 2. Here, xe2x80x9cdiffraction grating depthxe2x80x9d means the extent of the spatial modulation of the diffraction grating, and for a refractive index-type diffraction grating for example, it means the size of the spatial modulation of the refractive index.
FIG. 15 illustrates the diffraction efficiency of zero-order diffraction light as a function of the grating depth of the diffraction element 2 (line X) and the diffraction efficiency of xc2x11-order diffraction light as a function of the grating depth of the diffraction element 2 (line Y). As becomes clear from FIG. 15, an increase of the diffraction efficiency of xc2x11-order diffraction light leads to a decrease of the diffraction efficiency of zero-order diffraction light. This is a direct consequence of the law of the energy conservation.
Consequently, it was not possible to increase the light amount for the main beam and the sub-beams and to enhance the S/N ratio for both.
Moreover, conventional optical devices have the drawback that a tilt in the track direction of the information recording medium 6 causes an offset for the tracking error signal obtained by differential calculation from the sub-beam spots on the information recording medium 6. Such an offset may be caused by multiple reflections between the end face of the semiconductor laser element 1, the diffraction grating 2, the holographic element 3, and the information recording medium 6.
This mechanism is explained referring to the example shown in FIG. 16. The orientations of the X-axis, the Y-axis and the Z-axis in FIGS. 16A and 16B are the same as the respective orientations of the X-axis, the Y-axis and the Z-axis in FIG. 13. To keep the drawing simple, the semiconductor laser element 1, the diffraction grating 2, and the information recording medium 6 are the only structural elements shown in this drawing. The recording face of the information recording medium 6 is tilted with respect to the horizontal plane (indicated by a dashed line) by an angle xcex4 around the X-axis. In FIG. 16, points A and B denote points of the laser light irradiated onto the information recording medium 6, point C denotes the point of emission of the laser light, and point D denotes a point of light returning from the information recording medium 6 on the end face of the semiconductor laser 1.
As is shown in FIG. 16A, the light that is emitted from point C of the semiconductor laser element 1 is reflected at point B on the information recording medium 6, diffracted at the diffraction grating 2, and returns to point D at the end face of the semiconductor laser element 1. There, the light is reflected, passes the diffraction grating 2 again, and reaches point A at the information recording medium 6 (resulting in the light path 1: C  B  D  A). Reflected several times in this manner, the light may reach a photodetector group 7 (not shown in this drawing). As a result, a phase difference results, caused by the difference of the light path length of the light beam reaching the photodetector group 7 following the original path, and the two light beams may interfere with each other.
As is shown in FIG. 16B, the light emitted from the emission point C of the semiconductor laser element 1 is reflected at point A on the information recording medium 6, passes the diffraction grating 2, and returns to point D at the end face of the semiconductor laser element 1. There, the light is reflected, passes the diffraction grating 2 again, and reaches point A at the information recording medium 6 (resulting in the light path 2: C  A  D  A). Also in this case, the light, which has been reflected several times in this manner, may reach the photodetector group 7 (not shown in this drawing). As a result, a phase difference results, caused by the difference of the light path length of the light beam reaching the photodetector group 7 following the original path, and the two light beams may interfere with each other.
The degree of this interference (i.e. the interference strength) changes with the phase difference, which depends on the tilt angle xcex4 in the track direction of the information recording medium 6. Consequently, there is the problem that the signal strength based on the sub-beam spots varies, and there is an offset in the tracking error signal.
Similarly, interferences due to multiple reflections between the main beam and the sub-beams may occur, and lead to the problem that not only the S/N ratio of the reproduction signal but also the S/N ratio of the tracking error signal decreases.
It is an object of the present invention to provide an optical element, wherein the light amount of both the main beam and the sub-beams can be increased without increasing the light emission of the semiconductor laser element 1, and the S/N ratio of the main beam and the sub-beams can be enhanced.
It is another object of the present invention to provide an optical device, wherein an offset of the tracking error signal is reduced by suppressing multiple reflections between the information recording medium and the optical components used for the optical device, and the S/N ratio of the reproduction signal or the tracking error signal is increased by suppressing interference between the main beam and sub-beams.
An optical device in accordance with the present invention comprises:
a light-emitting element;
a diffraction grating for splitting light emitted from said light-emitting element into a plurality of beams, the diffraction grating comprising a first grating region and a second grating region with a diffraction efficiency that is different from a diffraction efficiency of the first grating region; and
a focussing member for focussing light that has passed through the diffraction grating.
In the diffraction grating in the optical device of the present invention, the diffraction efficiency of the first grating region for transmitting the main beam is independent from the diffraction efficiency of the second grating region for transmitting a sub-beam, which increases the light utilization efficiency.
Moreover, by setting the diffraction efficiency of the first and the second grating region such that the diffraction efficiency of xc2x11-order diffraction light of the first grating region for transmitting the main beam is lower than the diffraction efficiency of xc2x11-order diffraction light of the second grating region for transmitting the sub-beams, the diffraction grating in the optical device of the present invention can suppress multiple reflections between the information recording medium and the optical components used in the optical device. As a result, the offset of the tracking error signal is reduced. Moreover, interference between the main beam and the sub-beams can be suppressed. The synergy of this effect and the above-noted effect of increasing the light utilization efficiency enhances the S/N ratio of the reproduction signal and the tracking error signal even more.